The present invention relates to reception of digital signals and more particularly to systems and methods for synchronization.
To successfully recover data from a received signal, it is necessary for the receiver to detect the presence of the received signal and it is often necessary for the receiver to estimate values for various parameters of the received signal that cannot be precisely known in advance. Such parameters may include, e.g., the received amplitude of the signal, the carrier frequency of the signal, response characteristics of a communication channel used to transport the signal, the timing of transitions between successive symbols or bits encoded onto the signal, etc. In many situations, reliable data recovery is impossible without accurate estimation of one or more of these parameters.
To allow the receiver to determine such parameters and detect signal presence, many data communication schemes provide for a so-called preamble to be transmitted prior to any data. The preamble contents and structure are chosen to facilitate detection of the beginning of the frame and receiver estimation of characteristics such as frequency, timing, amplitude, etc. The present discussion will focus on detection of the frame start and establishment of timing synchronization.
In many communication systems, the preamble consists of repetitive groups of symbols, usually called short symbols. The short symbols may be as simple as alternating ones and zeros or as complex as short pseudorandom sequences. The presence of the short symbols is typically determined by simple detection of the received energy. Symbol synchronization is then determined by using inter-symbol transitions to derive a symbol clock by employing a phase-lock oscillator. This approach works well for high signal to noise ratio systems but performs poorly for weak signals such as are often found in wireless communication systems.
It is desirable to reliably detect the short symbols as early as possible with maximum reliability, even under conditions of noise, interference, and/or channel-induced distortion, in order to aid subsequent receiver functions, such as setting automatic gain control, estimating channel characteristics, and deriving carrier/symbol synchronization. It is also desirable to establish accurate and reliable timing synchronization under these conditions.